RE: Control Valves Wide Open

"running CVWO"----I always liked that phrase. throw in with "top heater out" and I am back in my glory days! (steam turbine commissioning)

Good question and I also hope there is a good discussion to follow.I have been away from the industry for a few years now and I never worked Combined units, only "normal" boiler/turbine plants.

the thing I have trouble understanding with this new combine plant controls is the ability to regulate grid frequency. I worked many normal power plants that were not allow (paid a high penility) if they operated with CVs near wide open such that they could not provide expected narrow range instenous MW/freq response.

I was under the inpression that combine steam turbines operated CVWO once one line and thus thier output followed HRSG pressure which followed combustion turbine firing rate.

RE: Control Valves Wide Open

RE: Control Valves Wide Open

Well, they (the combined cycle steam turbines) are smaller than almost all later coal fossil steam turbines. A 2 GT + 1 ST is often 225 Meg-Watt + 225 Meg-Watt + a 180-200 Meg-watt steamer. A 600-750 Meg steam coal plant is likely to only have one medium size turbine, rather than 3x small ones.

So the rapid load changes at the transformer yard are easiest to take up by adjusting either (or both) of the gas turbines, rather than the "free" energy of the steam turbine. The steam turbine, running on "free heat" from the two GT HRSG's costs no additional fuel to heat its steam generator. So, naturally, the operators want to run the steamer as wide open as possible, as long as both GT's are in their permitted zone of 45% - 100% power output. Few GT's run within their enviro permits below 45% max power, and their control circuit starts misbehaving below 20% max load. SO, the operators will try to lower both GT's down towards 45% rather than throttle the ST.

RE: Control Valves Wide Open

Here's something I first posted when I joined eng-tips that may be of help...

"It is very convenient to think of combined cycle power plants in 'N minus 1' terms. For instance: consider a hypothetical 'three over one' CCPP that consists of three CT's rated at 4 MW and one ST rated at 6 MW, for a total plant output of 18 MW derived from four units. Trying to dispatch the CT's and ST individually poses problems, since their outputs are interdependent. Such plants will instead bid into the market as a plant consisting of three 'pseudo-units' of 6 MW each. There are of course limitations to doing this, mostly due to the time lag involved with raising steam and synchronizing and loading the ST, and whether or not supplementary duct firing is used; but on the whole this works reasonably well."

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